The present invention relates to methods and apparatus for high resolution measurement of one or more characteristics of earth formations traversed by a borehole, and more particularly to methods and apparatus for high resolution measurement of one or more characteristics of earth formations traversed by a borehole for determining the dip and azimuth of these formation beds.
One of the most valuable aids in the exploration for oil and gas is the dipmeter log, which provides positive structural and stratigraphic information for both exploration and development drilling programs. Advances in dipmeter tool design, magnetic taping, machine computerization, and interpretation methods make it possible to recognize such features as structural dip, faults, unconformities, bars, channels, and reefs. In addition, the direction of sedimentation and of pinchouts can be estimated. When combined with data from other wells, dipmeter information helps to establish the overall structural and stratigraphic picture of the area under study.
The focussed current type of dipmeter has been particularly well received by the wireline logging industry for use in logging boreholes drilled with conductive drilling fluids. Focussed current dipmeter tools employ at least three pads and commonly four, each of which comprises one or more electrodes for emitting a focussed current beam into the adjacent formation. The current flow at each electrode is proportional to the conductivity of the adjacent formation. Focussed current dipmeters are described in U.S. Pat. No. 3,060,373, issued Oct. 23, 1962 to Doll; U.S. Pat. No. 4,251,773, issued Feb. 17, 1981 to Cailliau et al.; and U.S. Pat. No. 4,334,271, issued June 8, 1982 to Clavier. These are able to achieve good vertical resolution at reasonable logging speeds, the micro-resistivity sensors used on some of these tools being capable of resolution to as fine as 0.2 inch.
The great amount of data acquired by dipmeters, and especially the high resolution focussed current dipmeters, is advantageously exploited by the use of computers. For example, suitable computer implemented correlation techniques are described in U.S. Pat. No. 4,348,748, issued Sept. 7, 1982 to Clavier et al., and U.S. Pat. No. 4,355,357, issued Oct. 19, 1982 to Chan. Improved dip determinations often can be obtained by use of other computer-implemented techniques, such as that described in U.S. Pat. No. 4,453,219, issued June 5, 1984 to Clavier et al.
Other types of dipmeters have been proposed for use in boreholes drilled with conductive drilling fluids, including the electrical-toroidal type described in U.S. Pat. No. 2,987,668, issued June 6, 1961 to Gondouin. None of them has achieved the popularity of the focussed current tools.
Unfortunately, electrical dipmeters, including the focussed current type, are not altogether satisfactory for use in boreholes which have been drilled with a nonconductive fluid such as air or an oil-based mud. Electrical dipmeters require a conductive medium to permit the flow of current from the electrode system into the formation. This conductive medium is not present in boreholes drilled with air or an oil based mud.
Various approaches employing pad-mounted electrodes have been taken to obtain dip information in wells drilled with nonconductive drilling fluids. One approach, which is exemplified by U.S. Pat. No. 2,749,503, issued June 5, 1956 to Doll, and more recently by U.S. Pat. No. 3,973,181, issued Aug. 3, 1976 to Calvert, uses high frequency electromagnetic energy to measure the capacitive coupling of an electrode to the formation. Another approach, described in an article by Fons entitled "New Dipmeter Tool Logs in Non-conductive Mud," The Oil and Gas Journal, Aug. 1, 1966, pp. 124-26, advocates the use of monoelectrode contact knife-like electrodes to make direct contact with the formation.
Other approaches to obtain dip information in wells drilled with nonconductive drilling fluids dispense with electrodes altogether. Acoustic techniques employing pad-mounted acoustic transducers are taught in, for example, U.S. Pat. No. 3,376,950, issued Apr. 9, 1968 to Grine; U.S. Pat. No. 3,526,874, issued Sept. 1, 1970 to Schwartz; and U.S. Pat. No. 3,564,914, issued Feb. 23, 1971 to Desai et al. An electromagnetic wave logging dipmeter is disclosed in U.S. Pat. No. 4,422,043, issued Dec. 20, 1983 to Meador.
In addition, techniques based on the principal of induction logging have been proposed for measuring dip by the use of either mandrel-mounted coils or pad-mounted coils. In conventional induction logging, such as disclosed in U.S. Pat. No. 2,582,314, issued Jan. 15, 1952 to Doll, oscillating magnetic fields formed by one or more energized induction coils induce currents in the formation around the borehole. These currents in turn contribute to a voltage induced in one or more receiver coils through a secondary magnetic fluid. The voltage component of the received signal that is in phase with respect to the transmitter current, known as the R-signal, is approximately proportional to formation conductivity.
When operating a mandrel tool in a borehole traversing a homogeneous medium, ground current flow loops arise which coincide with the primary electric field induced by the primary magnetic field of the transmitter. Hence, the ground loops are coaxial relative to the receiving and transmitting coils and the borehole. Under-certain conditions of the surrounding earth formations, however, such as dipping beds or fractures, the average plane of the ground current flow loops vary from this coincident alignment. The phenomena is exploited in the mandrel-type induction dipmeter. In one early mandrel induction dipmeter, a coil array is mechanically rotated to produce modulation components in the receiver signals at the frequency of rotation of the coil array. The modulation components are processed to obtain indications of the dip, dip azimuth and/or anisotropy. More recently, techniques have been proposed which utilize mechanically passive induction coil arrays to obtain measurements of formation dip, dip azimuth, and/or anisotropy. Systems of this type are taught in, for example, U.S. Pat. No. 3,808,520, issued Apr. 30, 1974 to Runge; U.S. Pat. No. 4,302,723, issued Nov. 24, 1981 to Moran; and U.S. Pat. No. 4,360,777, issued Nov. 23, 1982 to Segesman.
Other induction techniques use pad-mounted field generating and sensing transducers to measure such characteristics as conductivity, magnetic susceptibility, and dielectric constant, as well as the dip of earth formations. An early system is described in U.S. Pat. No. 3,388,323, issued June 11, 1968 to Stripling. The Stripling apparatus comprises three circumferentially spaced sensors which are urged against the borehole wall. A composite field comprising a primary magnetic field and a secondary magnetic field is created and sensed by each sensor. Phase separation is applied to the sensed signal to obtain measurements of magnetic susceptibility and electrical conductivity. The sensor of the Stripling apparatus comprises a coil wrapped around a core of high-permeability material to increase the flow of magnetic flux through the coil. The coils have a length of about three inches and a diameter of about one-half inch. The axes of the coils are tangential to a circle lying in a plane normal to the tool axis. Separate transmitting and receiving coils are contemplated as well. The apparatus operates at frequencies under 60 kHz.
A pad configuration intended to reduce sensitivity to borehole diameter and borehole fluid conductivity is disclosed in U.S. Pat. No. 3,539,911, issued Nov. 10, 1970 to Youmans. The pad comprises a pair of transmitter coils, said to be wound in series opposition, mounted within the pad at an acute angle from the longitudinal axis of the elongated sonde, and a receiver coil mounted substantially parallel to the longitudinal axis between the transmitter coils. The mounting angles of the transmitter coils are chosen to provide what is said to be a desired asymmetrical field of investigation. The apparatus operates at about 20 kHz, and both in-phase or out-of-phase detection techniques are contemplated. The axial distance between the axes of the transmitter and receiver coils are said to influence the investigative mode, and mutual balance of the coil configuration is said to be attained by adjusting that distance.
More recently, U.S. Pat. No. 4,019,126, issued Apr. 19, 1977 to Meador disclosed an apparatus intended to avoid the temperature and pressure sensitivity of the aforementioned Stripling apparatus. Meador teaches that the sensing coil of an induction dipmeter arm may be constructed without a high permeability core, which is quite temperature and pressure sensitive. The coil proposed by Meador comprises two turns of one-eighth inch diameter copper wire, each turn being approximately three-quarters of an inch by three-eighths of an inch. Meador also teaches that two separate coils may be employed in each pad, one coil being the transmitter and the other being the receiver. The coil is arranged with its longitudinal axis parallel to the axis of the sonde. The coil is coupled with a capacitor to form a tank circuit, which is connected to an oscillator circuit. The operating frequency is said to be in the range of preferably between 50 MHz and 200 MHz, with satisfactory operation at lower frequencies as well. The Meador apparatus is intended to measure resistivity and dielectric constant.
The pad-mounted induction dipmeter systems generally have been disappointing. Some of the techniques are sensitive to borehole diameter and fluid conductivity, or to borehole temperature and pressure. Moreover, some of the systems themselves are not highly sensitive to the very parameters they are intended to measure, which is particularly troublesome when effects resulting from temperature, pressure, alignment inaccuracies, and operation instabilities contribute to the detected signal.